P0LTPIIA8E CURRENTS.

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In some instances mechanical appliances have been utilized forthis purpose. In others special and complicated forms of motorhave been constructed. Mr. Tesla has discovered a much moresimple method or plan of operating synchronizing motors, whichrequires practically no other apparatus than the motor itself. Inother words, by a certain change in the circuit connections of themotor he converts it at will from a double circuit motor, or suchas have been already described, and which will start under theaction of an alternating current, into a synchronizing motor, orone which will be run by the generator only when it has reacheda certain speed of rotation synchronous with that of the genera-tor. In this manner he is enabled to extend very greatly the ap-plications of his system and to secure all the advantages of bothforms of alternating current motor.

The expression “ synchronous with that of the generator,” isused here in its ordinary acceptation—that is to say, a motor issaid to synchronize with the generator when it preserves a certainrelative speed determined by its number of poles and the numberof alternations produced per revolution of the generator. Itsactual speed, therefore, may be faster or slow'er than that of thegenerator; but it is said to be synchronous so long as it preservesthe same relative speed.

In carrying out this invention Mr. Tesla constructs a motorwhich has a strong tendency to synchronism with the generator.The construction preferred is that in which the armature is pro-vided with polar projections. The field-magnets are wound withtwo sets of coils, the terminals of which are connected to a switchmechanism, by means of which the line-current may be carrieddirectly through these coils or indirectly through paths bywhich its phases are modified. To start such a motor, the switchis turned on to a set of contacts which includes in one motorcircuit a dead resistance, in the other an inductive resistance, and,the two circuits being in derivation, it is obvious that the differ-ence in phase of the current in such circuits will set up a rotationof the motor. When the speed of the motor has thus beenbrought to the desired rate the switch is shifted to throw themain current directly through the motor-circuits, and althoughthe currents in both circuits will now be of the same phase themotor will continue to revolve, becoming a true synchronousmotor. To secure greater efficiency, the armature or its polarprojections are wound with coils closed on themselves.